1. Field of the Invention
The invention relates generally to display pixels and methods of driving the same. More particularly, the invention relates to a pixel representing a gradation based on frequency characteristics of an organic light emitting diode, an organic light emitting display comprising the same, and a driving method thereof.
2. Discussion of Related Art
Recently, various flat panel displays have been developed as substitutes for a cathode ray tube (CRT) display due to their bulkiness and excessive weight. Types of flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic light emitting display (LED).
An organic light emitting display emits light independently using recombination of an electron and a hole, and is classified into two types: an inorganic light emitting display comprising an inorganic emission layer, and an organic light emitting display comprising an organic emission layer. The organic light emitting display can also be referred to as an electroluminescent display.
In contrast to a passive type display that requires a separate light source like the LCD, an organic light emitting display has an advantageously fast response time like a cathode ray tube (CRT) display.
FIG. 1 is a circuit diagram of a pixel provided in an organic light emitting display. Referring to FIG. 1, the exemplary organic light emitting display comprises a plurality of pixels 11 positioned at an intersection region of a scan line Sn and a data line Dm. An individual pixel 11 is selected when a scan signal is applied to a scan line Sn, and the selected pixel 11 emits light in response to a data signal applied to the data line Dm.
Each pixel 11 comprises a first power source line VDD, a second power source line VSS, an organic light emitting diode (OLED), and a pixel circuit 40.
The OLED comprises an anode electrode connected to the pixel circuit 40, and a cathode electrode connected to the second power source line VSS.
The organic light emitting diode comprises an emitting layer, an electron transport layer, and a hole transport layer, which are interposed between the anode electrode and the cathode electrode. Additionally, the organic light emitting diode may comprise an electron injection layer, and a hole injection layer. In such an organic light emitting diode, when voltage is applied across the anode electrode and the cathode electrode, electrons generated from the cathode electrode are moved to the emitting layer via the electron injection layer and the electron transport layer, and holes generated from the anode electrode are moved to the emitting layer via the hole injection layer and the hole transport layer. The electrons from the electron transport layer and the holes from the hole transport layer are then recombined in the emitting layer, thereby emitting light.
Referring again to FIG. 1, the pixel circuit 40 comprises a first transistor M1, a second transistor M2, and a capacitor C. The first and second transistors M1 and M2 are of a p-type metal oxide semiconductor (PMOS) field effect transistor (FET). In the illustrated example, the second power source VSS has a voltage level lower than that of first power source VDD, where the second power source VSS may be coupled to ground.
The first transistor M1 comprises a gate electrode connected to the scan line Sn, a source electrode connected to the data line Dm, and a drain electrode connected to a first node N1. In the illustrated example, the first transistor M1 supplies the data signal from the data line Dm to the first node N1 in response to the scan signal transmitted through the scan line Sn.
The capacitor C stores voltage corresponding to the data signal transmitted to the first node N1 through the first transistor M1 while the scan signal is supplied to the scan line Sn, and then maintains the second transistor M2 to be turned on for one frame when the first transistor M1 is turned off.
The second transistor M2 comprises a gate electrode connected to the first node N1 to which the drain electrode of the first transistor M1 and the capacitor C are commonly connected, a source electrode connected to the first power source line VDD, and a drain electrode connected to the anode electrode of the organic light emitting diode OLED. In operation, the second transistor M2 adjusts the intensity of current on the basis of the data signal supplied from the first power source line VDD to the organic light emitting diode OLED. Thus, the OLED emits light based on the current supplied from the first power source line VDD through the second transistor M2.
The pixel 11 thus operates as follows. When a low state scan signal is transmitted to the scan line Sn, the first transistor M1 is turned on. Then, the data signal is supplied from the data line Dm to the gate electrode of the second transistor M2 via the first transistor M1 and the first node N1. At this time, the capacitor C stores a voltage corresponding to the voltage difference between the gate electrode of the second transistor M2 and the first power source line VDD.
In response to the voltage applied to the first node N1, the second transistor M2 is turned on and supplies a current, corresponding to the data signal, to the OLED. Thus, the OLED emits light based on the current supplied from the second transistor M2, and thereby displays an image.
When a high state scan signal is transmitted to the scan line Sn, the voltage that corresponds to the data signal is stored in the capacitor C and maintains the second transistor M2 to be turned on for one frame. Accordingly, the organic light emitting diode OLED emits light for one frame, thereby displaying an image.
Further, the exemplary organic light emitting display may additionally comprise a compensation circuit (not shown) to compensate for the non-uniformity of the threshold voltages of a plurality of second transistors (e.g., the second transistor M2) due to a manufacturing process. Although the organic light emitting display comprising the compensation circuit may operate in an offset compensation manner or a current programming manner, there are still limitations to the display of an image with uniform brightness.